Embodiments of the inventive concept relate generally to electronic memory technologies. More particularly, embodiments of the inventive concept relate to semiconductor memory devices that can be electrically erased and programmed.
Semiconductor memories play an important role in a wide variety of modern electronic devices, ranging from satellites to consumer products. Consequently, advances in semiconductor memory technology can lead to significant improvements in a broad range of technical applications.
Semiconductor memory devices can be broadly classified into two categories based on whether or not they retain stored data when disconnected from power. These categories include volatile semiconductor memory devices, which lose stored data when disconnected from power, and nonvolatile semiconductor memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include static random access memory (SRAM) devices and dynamic random access memory (DRAM) devices. Examples of nonvolatile memory devices include various types of read only memory (ROM), such as MROM, PROM, EPROM, and EEPROM. There are also hybrid forms of memory comprising nonvolatile and volatile parts, such as nonvolatile RAM (nvRAM) devices.
Nonvolatile memory devices are commonly used to store programs, microcode, and other information in applications such as a computers, avionics, telecommunications, and consumer electronics. Many of these applications require the nonvolatile memory devices to have fast access and programming characteristics. Accordingly, certain nonvolatile memory devices have been developed with specific structures and logic circuits to optimize performance for application-oriented work.
Flash memory is a form of EEPROM that has achieved increasing popularity in recent years. Among other things, flash memory is relatively inexpensive and provides high performance and data storage capacity compared with other forms of nonvolatile memory. In addition, flash memory is resistant to physical shock, which has made it especially popular for use in mobile devices, such as cellular phones, digital cameras, netbook computers, and so on.
In flash memories, the memory cells are typically configured in one of two configurations, including a NOR configuration, and a NAND configuration. Flash memories having these configurations are referred to as NOR flash memories and NAND flash memories, respectively. The NOR configuration tends to provide faster access speed, but lower storage capacity, compared with the NAND configuration. Accordingly, NOR flash memories are often used to store information requiring fast access, such as code, while NAND flash memories are generally used to provide mass data storage capability for information such as multimedia information, data files, and so on.
Most flash memories store one bit of data per memory cell. However, flash memories have been developed to store more than one bit of data per memory cell. Flash memories that store more than one bit of data per memory cell are referred to as multi-level cell (MLC) flash memories.